1. Technical Field
The present invention relates to electric double layer capacitors having large capacitance and high power, and in particular, relates to a technique to prevent gas generation due to decomposition of the electrolyte solution used in the capacitor.
2. Background Art
An electric double layer capacitor has characteristics such as long service life, high cycling characteristics, characteristics of charge and discharge with large current, and wide range of operating temperatures since there are no chemical reactions during charge and discharge of the capacitor as there are in a conventional secondary battery. Therefore, this type of capacitor is attracting much attention as a new type of storage battery or as a driving power supply for automobiles and devices. In particular, electric double layer capacitors having large capacity and high power are being developed.
As an example of such an electric double layer capacitor, a button-type electric double layer capacitor is shown in FIG. 1. As shown in FIG. 1, the capacitor 1 includes a case 2, a pair of polarizing electrodes 3 and 4 contained in the case 2, a spacer 5 disposed between the electrodes, and electrolyte solution filled in the case 2. The case 2 includes an aluminum body 7 having an opening part 6 and an aluminum cover plate 8 which closes the opening part 6. A part between the outer circumference of the cover plate 8 and inner circumference of the body 7 is sealed with a sealing material 9. The polarizing electrodes 3 and 4 include a mixture of an activated carbon for an electrode, conductive filler, and binder.
Conventionally, as the electrolyte solution used as a material of the electric double layer capacitor, a water-based electrolyte solution and a non-water-based electrolyte solution can be mentioned. In particular, since high voltage use is required to improve energy density of the electric double layer capacitor, the non-water-based electrolyte solution which can be charged and discharged at relatively high voltage has been widely used. As the non-water-based electrolyte solution, a solution in which various conditions such as low-temperature property, solubility of salts, dielectric constant, safety, electrolyte solution decomposition property, boiling point, cost and the like are improved is required. As a material which meets these requirements, an electrolyte solution in which propylene carbonate is mainly used as a solvent and a quaternary ammonium salt is added as a supporting salt, may be mentioned.
However, in the electric double layer capacitor having an electrolyte solution containing propylene carbonate and an alkali activated carbon, in the case in which it is charged and discharged repeatedly under high voltage, the electrolyte solution is gradually decomposed by an electric current and gas is generated. The pressure inside the capacitor is increased by the gas generation, and the capacitor case may be deformed or broken. Furthermore, as a result of consumption of the electrolyte solution by the electrolysis, capacitance may be deteriorated.
To solve such problems, methods in which various types of materials are added to reduce decomposition of the electrolyte solution have been suggested. Practically, a non-water-based electrolyte solution containing γ-butyrolactone or γ-valerolactone in which electrolysis is reduced (see Japanese Unexamined Patent Application Publication No. 2001-217150), a non-water-based electrolyte solution containing fluorobenzene (see Japanese Unexamined Patent Application Publication No. 2004-6803), a non-water-based electrolyte solution containing diphenyls (see Japanese Unexamined Patent Application Publication No. 2004-146610) and the like are disclosed.
However, in these solvents, the effect of restraining gas generation is insufficient. In addition, dehydration and purification of the additives are required to add the additives to the electrolyte solution, and the cost is increased by the solution containing multiple components.